The present invention relates to a method for determining the total refractive power of the cornea of an eye, and is based on a combination of keratometric or topographical measurements with the measured values obtained from depth scans or sectional images. The total refractive power of the cornea is particularly significant also with regard to the calculation and selection of intraocular lenses.
According to the known state of the art, numerous solutions are known for this purpose.
For highly precise measurement in the form of depth scans, solutions have established themselves in the state of the art, which are based on the method of optical coherence tomography (OCT), partial coherence interferometry (PCI) or the like.
The fundamental principle of the OCT method is based on white light interferometry and compares the running time of a signal using an interferometer (generally a Michelson interferometer). In this connection, the arm having the known optical path length (=reference arm) is used as a reference for the measurement arm. The interference of the signals from both arms yields a pattern from which the relative optical path length within an A scan (individual depth signal) can be read out. In the one-dimensional raster methods, the beam is then guided transversally in one or also in two directions, analogous to the ultrasound technique, thereby making it possible to record a two-dimensional B scan or a three-dimensional tomogram (C scan). In this connection, the amplitude values of the individual A scans are typically represented as logarithmic gray-scale or false-color values.
In contrast to this, sectional images can be produced using Scheimpflug cameras or also slit lamps.
A Scheimpflug camera is based on adherence to what is called the Scheimpflug rule, according to which rule the image plane, lens plane, and focal plane must intersect in a common straight line, so that the entire object plane is imaged with maximal focus. With regard to the implementation of sectional images for determining the total refractive power of the cornea of an eye, the advantage of the camera is rooted in the fact that the entire object plane of the section through the cornea is imaged with sharp focus, and the recorded images do not contain any blurring.
A slit lamp (also: slit lamp microscope) is one of the most important ophthalmological examination instruments, with which an eye doctor or optician can inspect the eyes stereoscopically. The examining person has the possibility of directing a sharply delimited slit-shaped beam of light, the width of which can be changed, onto the eye. At the same time, he/she has the possibility of observing the eye through the incident light microscope. The enlargement of the microscope is variable, in most of the devices, and usually ranges from 6 times to 30 times.
By means of different lighting methods (diffuse, direct, focal, indirect, regressive, lateral, etc.) and variable light slit widths, it is possible to inspect almost all the anterior, central, and posterior sections of the eye, all the way to areas of the retina situated far in the periphery. For many examinations, additional aids, such as, for example, a three-mirror contact lens, are required. Most modern slit lamps have a digital camera for documenting findings on film or in photographs.
If, however, additional measurement variables are needed, these can be determined, for example, from keratometric or topographical recorded images of the eye.
It is true that these further measurement variables and the OCT measured values can be measured by application of different devices, but integration of the measurement both of OCT and of the further measurement variables in one device allows easier handling, for example only one-time alignment of the device relative to the patient, and improved lateral registration of the OCT measured values with the further measured values.
In a first group of solutions, the different images are recorded sequentially, i.e., one after the other.
An example is shown by US 2005/0203422 A1, which shows a combination system of a keratometer and OCT tomography. In order to separate the two modalities from one another, separation in terms of time is also proposed here.
A further example is the IOLMaster from the Carl Zeiss company. This is a combination device that determines the keratometry, the axial length by way of PCI (partial coherence interferometry), and the anterior chamber depth by way of slit illumination and image detection, as well as further parameters of the eye, such as what is called the white-to-white distance.
In all of these measurements that take place sequentially, the time expended for the measurements is greater. It is furthermore disadvantageous that the different measurements of OCT and ultrasound or keratometry could take place at slightly different locations because of possible eye movements. In general, reproducibility of the measurement is accordingly difficult to implement.
In a second group of solutions, the different images are recorded at the same time, and for this, the measurement systems must have corresponding optical separation at their disposal.
As a further example, a combination system composed of a keratometer and axial length measurement by means of PCI is described in US 2005/0018137 A1. In this connection, the separation of the two modalities is implemented by beam splitting by means of polarization separation.
The document US 2005/0203422 A1, which has already been mentioned above, also mentions separation of the modalities (by means of OCT and keratometry), by means of a dichroic beam splitter, as an alternative to sequential measurement.
In all of these examples, optical separation of the different measurement systems takes place either by means of the use of different wavelengths or by way of additional optical elements that prevent the measurement systems from reciprocally influencing one another.
It is a disadvantage in the techniques described above that the traditional methods of keratometry and topography measure only the radius Ra of the anterior cornea side, and determine the total refractive power KKer of the cornea, including the optical effect of the posterior side, from this measurement by approximation. According to the literature [7], the total refractive power KKer is calculated as follows:
                    KKer        =                              nK            -            1                    Ra                                    (        0        )            
The assumptions concerning cornea thickness and anterior/posterior side ratio, as well as their indices of refraction, are implicitly included in the value nk.
Other approaches measure the total refractive power of the cornea without assumptions concerning the anterior/posterior side ratio and/or cornea thickness, by means of using OCT or a combination of OCT and topography. Good reproducibility of the measurement of the cornea radii by means of OCT is difficult to achieve. In particular, high scanning speeds and resolutions are required due to possible eye movements during the measurement. For a correct measurement of the radii, precise calibration of the OCT (including scanner) is required. This also holds true in combination with topography.
For selection of an intraocular lens (IOL) by means of the IOL calculation formula, the empirical knowledge of the ULIB database is frequently accessed, in practice. For this purpose, it is necessary that the measured total refractive power of the cornea, on the average of the population having normal eyes, agrees with the measured values of the keratometer of the IOLMaster.
This is not the case for existing approaches for determining the total refractive power of the cornea of an eye, or at least has not been proven.